The presently disclosed embodiment relates to a method for varying an oscillation frequency of a high frequency oscillator such as an electron tube and a solid-state oscillator oscillating a high frequency wave such as a micro wave, and in particular relates to a method for varying an oscillation frequency of a high frequency oscillator such as a magnetron, in which the structure of the oscillator is simple and the oscillation frequency can be varied with an electrical signal from the outside of the oscillator.
FIG. 8 shows a basic structure of a magnetron which is a kind of an electron tube, for varying a reactance of a switching element with a bias voltage from the outside of the magnetron (for example, see Patent Document 1). In the magnetron, an anode, in which a cathode 1 is disposed at the center thereof, is provided outside the cathode 1 concentrically therewith. The anode is composed of a cylindrical anode shell 2, a plurality of anode vanes 3 extending from the inner wall of the anode shell 2 toward the cathode 1 so as to divide the inside of the anode shell 2 into plural portions in a circumferential direction, and straps 4 connecting the alternate anode vanes. This anode functions not only as a positive electrode against the cathode 1 but also as a resonator determining the oscillation frequency. Small cavities separated by the anode vanes form resonant cavities resonating at a frequency close to the oscillation frequency.
The straps 4 are provided in order to best stabilize the stable π-mode oscillation of the magnetron, and a line-shaped metal conductor is used as the strap. The anode vanes 3 serving as partitions of the resonant cavity segmented into a plurality of spaces as mentioned above are connected alternately with the strap. In a magnetron having such a structure, its oscillation frequency is determined by a reactance of the resonant cavity and a reactance which is caused by the straps 4. In addition, in such a configuration of the magnetron as shown in FIG. 8, to vary a resonance frequency of the magnetron by applying a bias voltage from the outside of the magnetron, a through-hole 11 is formed in the anode shell 2 which is a wall of the resonant cavity, and a window 12 made of a low dielectric loss material such as ceramic or glass is provided to close the outer side of this through-hole 11 and to maintain a vacuum condition of the resonant cavities (magnetron tube) (that is, the resonant cavities are sealed from outside). A metal rod (rod-like metal) 14 is provided outside this window 12 to cover a part of the front of the window 12. One end of this rod 14 is supported on the anode shell 2 with a supporting member (metal) 16a in a state of being electrically insulated via an insulator 15, and serves as a terminal 14T to which a bias voltage is applied. Further, one end of the switching element 18 made of a PIN diode is connected to another end of the rod 14, and another end of this switching element 18 is electrically connected (short-circuited) to the anode shell 2 with a supporting member (metal) 16b. An electric field of the resonant cavities extends to the outside of the resonant cavities via the through-hole 11 and the window 12. Usually, when the bias current is not flowing, the switching element 18 is turned off and the rod 14 is placed apart from the electric potential of the anode shell 2, and thus, the extended electric field is not blocked and the resonance frequency increases above the frequency of the original resonant cavity. In other words, the reactance outside the tube acts on the reactance in the anode shell 2 which is the tube.
Subsequently, when the bias current is flowed and the bias voltage is applied between the anode shell 2 and the terminal 14T to turn on the switching element 18, the rod 14 is shot-circuited with the anode shell 2 in a high-frequency manner, and the switching element 18 and the rod 14 block the electric field extended from the window 12 while increasing the Rf resistance with the increase of the bias current. As a result, the oscillation frequency decreases as the bias current increases. As one of conventional methods, there is a method of coupling another resonator to the primary resonant cavity of the magnetron and changing the reactance of this another resonator to thereby changing the resonance frequency of the composite resonant cavity. However, the configuration of the presently disclosed embodiment is such that another resonator is not coupled for changing the resonance frequency and the resonance frequency of the single resonant cavity is changed by changing the electric field extended from the resonant cavity (coupling factor at the portion of the window 12) without providing another resonator.
FIG. 9 shows an another basic structure for varying an oscillation frequency of a magnetron being a kind of an electron tube by changing a reactance of a switching element using a bias voltage from the outside of the magnetron (for example, see Patent Document 2). The structure of the main part of the magnetron is the same as that of FIG. 8, and is composed of a cathode 1, an anode shell 2, anode vanes 3, and straps 4. A coaxial central conductor 17 is inserted into a resonant cavity of the anode shell 2 through a through-hole 11. A dielectric portion 25 covering the through-hole 11 is provided outside the through-hole 11 formed on the wall surface of the anode shell 2. This dielectric portion 25 is made of a dielectric material such as ceramic or glass, for example, and is mounted so as to be in a state of maintaining a vacuum of the magnetron tube.
In this anode shell 2, an end of the coaxial central conductor 17 is connected to a part of the anode structure such as the anode vane 3 for coupling with a reactance of the resonant cavity, and the other end passes through the dielectric portion 25 to be lead to the outside, and is connected to the switching element 18 via an external conductor 34. In other words, the dielectric portion 25 is placed between the coaxial central conductor 17 and the anode shell 2, and serves as an insulator of a dielectric member for a coaxial structure. A bias voltage is applied to the other end of this switching element 18. In other words, by connecting the other terminal of the bias to a point in which its electric potential in a direct flow is the same as that of the anode shell 2, the bias direct current flows through the switching element 18, external conductor 34, coaxial central conductor 17, anode vane 3 and anode shell 2 in this order. The current direction is determined in the case of using a PIN diode in the switching element 18 since it has a polarity. The bias voltage is applied in accordance with its polarity depending on the attachment direction of the switching element 18. In the case where the diode of the switching element 18 is replaced by a varactor diode, an oscillation frequency is variable within a wide range by reversing the bias direction since a change of a reactance by application of the bias voltage is large.
As mentioned above, it is known that there are electronic frequency tuning magnetrons, in which an oscillation frequency is varied by applying a bias voltage to a switching element 18 from the outside to change a reactance of the switching element 18, thereby producing an effect on a resonance frequency of a cavity of a magnetron being coupled to the switching element 18 in a high-frequency manner.
Additional background information may be found in Japanese publications JP 2009-277379 A and JP 2011-060591 A